NcAppNavigationSettings: Settings toggle button stil has `--pill` border radius

[st3iny]

The toggle button for the app navigation settings is still using the old --pill border radius.

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I tried to simply replace it with var(--border-radius-element, calc(var(--default-clickable-area) / 2)); but it somehow doesn't look right.

PR at https://github.com/nextcloud-libraries/nextcloud-vue/pull/5765 if you think this is good enough.